I knew it didn't feel right when I put in "aCS" for 1D: Windows runners (Saw "Window runners" and thought of air-conditioning. Idiotic, I know), but I never went back to check on it and, well, that, my friends, is how you end up Finished With One Error.
Apart from my own little problem, I really enjoyed this one. It's an unusual theme, with the HORIZON in the middle splitting answers about SEA and SKY. FLOCKOFBIRDS and SCHOOLOFFISH are so perfectly matched, and they're such normal expressions. And PLANE and CORAL aren't bad either. Overall, the theme gets an A. Hey, maybe I'll add that to the list of rankings that we (almost always) do here at Horace and Frannie and Colum. We already rank the entry at 1A - which today is problematic, because it's part of the theme. I believe we abstain on days when that happens. But the other thing we do is pick a favorite clue/answer pair (also non-theme-related). Today there's lots to choose from, including references to "Office Space," "The Wizard of Oz," "Happy Days," and grammar school (NOCUTS), but my favorite is 27A: Things spotted in a casino (DICE). I love that kind of cleverness.
As I said, there was a lot to like in here - KOOKS (7D: Nutcases), AVARICE (11D: One of the seven deadly sins), and the classic clue type 17A: Genius's head? (SOFTG) - which today didn't get me mostly because I didn't even see the clue until I started writing this review. There's a ton of small stuff in here, but none of it is too, too objectionable. And that SW stack is lovely, with HIPSTER, TRACHEA, EARHART, and, well... ok, SITONIT might skew a little old, but hey, I got it and I liked it, so... "Ayyyyyyy."
Overall, quite an enjoyable Tuesday.
p.s. After posting I read Mr. Collins' notes on this puzzle, and he mentions the unfortunateness of having Amelia EARHART down there in the water. OOH, that's EERIE. Too soon!